Saturated steam cooled by a HEX

[Renoyd]

120 Deg C saturated steam (198.53 Kpa(a))is cooled by a heat exchanger. The outlet condition is 80 Deg C and 175 kPa(a) (under-saturated from steam table). What is the physical process of the steam in the HEX? Is it from 120 C steam to 120 C water, then from 120 C water to 80 C water? Or anything else? I need to calculate the HEX duty, and I need to identify the physical process first. Please help. Thanks.

 

[katmar]

The vapor will condense to liquid at the saturation temperature corresponding to the pressure, and then the liquid water will be subcooled. But the thermodynamic path has no influence on the heat duty. The duty is simply the enthalpy at the starting condition less the enthalpy at the end condition - irrespective of how it got there.

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[25362]

In short: 2706.0-334.9 = 2371.1 kJ/kg.

 

[chicopee]

There is a number of scenarios that you can analyze and they all depend and the type of steam diagram you draw.
You can draw five basic steam diagrams which are: T-s,T-v,P-v,P-h,H-s. From anyone of these diagrams, you can determine possible routes along constant temperature, Pressure, specific volume, entropy and even enthalpy. Why along these constant variable? because many of the thermodynamics relationships are based on maintaining at least one of these variables constant.

 

[LOUDOG]

Not that your customer will probably care, but the flow inside you HEX could have saturated steam along with sub-cooled water, depending on the flow and arrangement of the HEX. Different flow regimes can produce funny results. But you dont need to know this to calculated your duty.

 

[Renoyd]

Thank you, everyone. It helps.